1. Field of the Invention
This invention relates to concentrated aqueous liquid detergent compositions containing an end-capped hydrophilic polymer as a deflocculating agent, such polymer being preferably terminated with an alkyl sulfide, alkyl sulfoxide or alkyl sulfone end-cap group. The liquid detergent compositions may also contain a high molecular weight cross-linked polyacrylic acid compound as a stabilizing agent to maintain the viscosity substantially constant during storage and prevent the continuous loss of viscosity over time, a characteristic behavior of certain concentrated detergent compositions.
2. Description of the Related Art
Heavy duty liquid detergents useful for machine washing of laundry are well known materials which have been described in a number of patents and in the literature. They are generally aqueous compositions comprising at least one or a compatible mixture of two or more detergent active surfactants selected from anionic, cationic, nonionic, zwitterionic and amphoteric species. Such compositions also generally contain detergency builder components and/or sequestering agents such as inorganic phosphates or phosphonates, alkali metal carbonates, alkali metal aminopolycarboxylates such as salts of nitrilotriacetic acid and salts of ethylenediamine-tetraacetic acid, alkali metal silicates, aluminosilicates, various zeolites and mixtures of two or more of these. Other components which may be present in such compositions include a clay material such as bentonite present as a fabric softener, optical brighteners, enzymes and their stabilizers, perfumes, colorants, antifoaming agents, e.g. silicone compounds, preservatives and like known additives.
A particular category of liquid detergents are the so called structured liquids comprising lamellar droplets (micelles) dispersed in an aqueous electrolyte phase. The lamellar droplets consist of an onion-like configuration of concentric bi-layers of surfactant molecules between which layers are trapped water or electrolyte solution. Such liquids may also contain suspended solids such as the water insoluble builders and clays referred to above.
There is a trend in the industry to provide detergent compositions having a higher concentration of active ingredients (payload), including surfactants. These include detergent concentrates containing about 10 to 25% by weight of surfactant and super concentrates containing from about 25 to 45% by weight surfactant. However, as the level of surfactant is increased, the volume fraction of lamellar droplets suspended is also increased, resulting in a diminished spacing between droplets. Contact of the suspended lamellar droplets with one another can lead to a congealing or flocculation phenomenon, resulting in a marked increase in the viscosity of the detergent composition due to formation of a network throughout the liquid. Liquids containing flocculated lamellar droplets are unacceptable because of phase separation and a difficulty in pouring such liquids from their containers.
One approach to enhance the stability of such compositions is the inclusion of minor amounts, e.g., 0.01 to 5% by weight, of a deflocculating polymer into the detergent formulation. For example, U.S. Pat. No. 5,147,576 discloses random interpolymers derived from hydrophilic monomers, such as acrylic acid, and also containing one or more copolymerized monomers having pendant hydrophobic side chains randomly dispersed along the polymer chain. Use of these interpolymers in detergent compositions is disclosed to hinder or prevent flocculation of lamellar surfactant droplets dispersed in the detergent, and thus enhance stability.
Hydrophilic polymeric materials have also been used in liquid detergent compositions as viscosity control agents. For example, U.S. Pat. No. 4,715,969 and its counterpart UK 2,168,717 disclose that the addition of less than about 0.5% by weight of a polyacrylate polymer, e.g. sodium polyacrylate, having a molecular weight from about 1,000 to 5,000, to aqueous detergent compositions containing primarily anionic surfactants will stabilize the viscosity of the composition and prevent a major increase in viscosity after a period of storage of the formulated composition. Also, EPO 301,883 discloses similar compositions containing from about 0.1 to 20% by weight of a viscosity reducing, water soluble polymer such as polyethylene glycol, dextran or a dextran sulfonate. U.S. Pat. Nos. 3,668,230; 3,839,405; 3,772,382; and 3,776,874 issued to Uniroyal, Inc. disclose alkyl sulfide, alkyl sulfoxide and alkyl sulfone terminated oligomers for use in emulsion polymerization. The oligomers are broadly stated to be useful as surface active agents, emulsifiers and thickeners.
EP 623670A describes the use of stabilizers in an aqueous surfactant composition to reduce the flocculation of systems containing a flocculable surfactant. The stabilizers are described as surfactants having a hydrophobic portion and a hydrophilic portion. The hydrophilic portion is typically a polymer linked at one end to the hydrophobic portion.
While the problems of phase separation and flocculation noted above which are frequently associated with concentrated liquid detergent compositions have been generally addressed in the prior art by the use of deflocculating polymers, there, nevertheless, remains for certain of the resulting liquid detergent compositions, depending on the particular composition and method of manufacture, the problem of a continuous viscosity "decay" or viscosity loss during storage eventually resulting in phase separation. Viscosity losses on the order of 40% or more over a four week period during storage are commonly observed in some compositions, and particularly at temperatures substantially above room temperature. For commercial concentrated liquid detergent products which typically have a target viscosity of from 2,000 to 8,000 cps, a decrease in viscosity of 40% or more during storage relative to its initial value represents a readily observable change in the pourability of the composition, a drawback which may adversely affect consumer acceptability.
Accordingly, one aspect of the present invention provides for the use of a high molecular weight cross-linked polyacrylic acid compound as a viscosity stabilizer for those concentrated liquid detergent compositions characterized by the above-described viscosity decay. As a general proposition, polyacrylic acid type polymers are well known, particularly in the machine dishwashing art, but, primarily for their thickening properties. Thus, for example, U.S. Pat. No. 5,053,158 to Dixit describes the use of high molecular weight cross-linked acrylic acid polymers as thickeners to provide the desired-thickening and viscous properties in a liquid automatic dishwasher detergent composition.
In U.S. Pat. No. 4,836,948, a cleaning composition in gel form is described for use in an automatic dishwasher. Certain desired viscoelastic properties of the gel are obtained by the use of a cross-linked polycarboxylate polymer, especially a cross-linked polyacrylic acid.
U.S. Pat. No. 4,715,969 to Rothanavibhata describes liquid detergent compositions which contain low molecular weight polyacrylate in amounts up to 0.5% to prevent the viscosity from increasing during storage to the extent that it interferes with the pourability of the liquid composition.
The use of linear polyacrylates having molecular weights above 4,500 is noted in the patent literature to be detrimental to the stability of built aqueous alkaline liquid compositions. In EP 322 946, for example, the patentee states that experimentation with polyacrylates of varying molecular weights has shown that for built alkaline liquid compositions containing a polyacrylate, the loss of physical stability becomes much worse as the molecular weight of the polyacrylate increases.
Accordingly, the prior art has yet to address itself to the general problem of viscosity loss over time which occurs in certain concentrated liquid detergent compositions and particularly at elevated temperatures, and has heretofore, been unaware of the beneficial effect which high molecular weight cross-linked polyacrylates have in stabilizing and substantially preventing the occurrence of such viscosity loss.